[IPV4] fib_trie: dump table in sorted order

It is easier with TRIE to dump the data traversal rather than
interating over every possible prefix. This saves some time and makes
the dump come out in sorted order.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>